Processing facilities, such as paper-making mills and material manufacturing mills, are typically managed using process control systems and quality control systems (QCS). Valves, pumps, motors, heating/cooling devices, and other industrial equipment and electronics typically perform actions needed to process materials in the processing facilities. Among other functions, the process control systems and QCS often manage the use of the industrial equipment in the processing facilities.
The process control system may comprise a Distributed Control Systems (DCS). The DCS is typically connected to and works in coordination with the QCS which generally include devices for measuring attributes of the product being manufactured (e.g. paper), and for sending control signals to the industrial equipment to adjust the quality of the product they are manufacturing.
Controllers are often used to control the operation of the industrial equipment in the processing facilities. The controllers typically monitor the operation of the industrial equipment, provide control signals to the industrial equipment, and/or generate alarms when malfunctions are detected. The field devices can include sensors (e.g., temperature, pressure and flow rate sensors), as well as other passive and/or active devices. Process controllers can receive process information, such as field measurements made by the field devices, in order to implement a control routine. Control signals can then be generated and sent to the industrial equipment to control the operation of the process and the manufacture of materials.
In the manufacture of paper, for example, a number of field devices are known to be utilized for measuring the moisture content and the temperature of the paper during the paper-making process. In the process, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric. Water in the paper drains by gravity and suction through the fabric. The web is then transferred to a pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal system.
A typical forming section of a paper-making system includes an essentially endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. The wet sheet is then transferred to the press section of the paper-making system where enough water is removed to form a sheet of paper. Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced.
On-line measurements made during the paper-making process generally include basis weight, moisture, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and thus minimizing the quantity of product that is rejected. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge.
Referring to FIG. 1, a known scanning device 17 shown is used to traverse the sheet material (e.g., paper 14) from edge to edge and take one or more property measurements of the sheet material during the paper-making process. Scanning device 17 includes multiple sensors S1, S2, and S3 that are arranged adjacent to one another. The sensors are distinct from one another since each sensor uses its own specific hardware to measure a specific property of the sheet material. Each specific property generally requires a different measurement technology. For example, S1 may be an Infra-Red (IR) sensor to measure moisture at location P1, S2 may be a nuclear element to measure basis weight at location P2, and S3 may be an optical unit for measuring another material property at location P3.
The scanning device 17 is generally placed at the “dry” end of the paper-making process where the sensors are not exposed to high temperatures or high humidity conditions, such as the extreme conditions associated with a “wet-end”, for example, near a steam box. The sensors S1, S2 and S3 of the scanning device 17 are on the dry end because their on-board electronics may not operate properly, or can be damaged, in the high heat and moisture environments at the “wet-end”. Furthermore, the sensors S1, S2 and S3 together are generally bulky in size and cannot easily fit, or be placed, at strategic measuring locations within the “wet-end”. Accordingly, the “dry-end” of the paper-making process is generally used for taking property measurements of the sheet material during the paper-making process.
A control system used to monitor the paper-making process can assess the paper properties measured at the different locations P1, P2 and P3 at the “dry-end”, and control process parameters based on these “final” measurements. Actuation of systems at the wet end, press, dryer, and finishing sections of a paper machine are typically all controlled by readings from the “dry-end” scanner at the reel. For example, it is conventional to measure the moisture content of sheet material upon it leaving the main dryer section, or measure the moisture content at the take-up reel employing the scanning sensors at the “dry-end”. The measurements at the “dry-end” can be used to adjust the machine operation of the paper-making process to achieve desired parameters and performance.
While “dry-end” measurement control has provided significant benefits to the paper manufacturing industry, it does have certain limitations. For instance, the traditional scanning device, such as scanning device 17 shown in FIG. 1, may not be able to accurately measure basis weight or caliper due to the differences in location (P1, P2 and P3) of the respective measurements on the paper 14. Although the sensors are proximate to one another, they are generally unable to measure discrete quality properties at a particular location simultaneously. Moreover, the scanning device may not be able to take measurements at different points in the process, for example, at the “wet-end” where extremely high temperature and moisture conditions exist, which as previously noted can result in damage to the receiving sensors (S1, S2 and S3).
The “dry-end” measurements are generally inadequate for capturing the changes of the paper at different process points in the paper-making process. Such incomplete process measurements can result in manufacturing deficiencies, such as lower quality and higher cost. Moreover, when measurements taken at the “dry-end” in the paper-making process are used to estimate measurements at the “wet-end”, errors can be introduced as a result of the estimation. For example, the measurements may not be from the same spot of paper, thus the errors may not be due to the same processes. Errors can also be generated due to machine speed inaccuracies, different measurement spot sizes, non-linear shrinkage and/or sheet wander. Furthermore, traditional scanning devices operating at the “dry-end” have difficulty separating densification in the press section from that in the dryer sections, and thus cannot generally provide accurate measurements of paper density.